The Portuguese Study Group of Thorium Dioxide's Side Effects has been conducting, since 1961, clinical, pathological and epidemiological research on the side-effects of Thorotrast when injected in human beings for diagnostic purposes, under the sponsorship of U.S. Public Health Service grants (RH 00039, EG 0068 and CA 14955). The results have been published in several papers and presented and discussed at various national and international scientific meetings. Due to its long experience and vast material on Thorotrast human toxicity, the Portuguese Study Group proposes to organize in Portugal, in June 1977, an International Conference on the Toxicity of Thorotrast and Other Alpha-Emitting Heavy Elements. Public awareness and concern regarding the development and use of transuranic elements in energy production and other fields is increasing, because of the increasing quantities of such elements being produced, used and distributed and the known or suspected carcinogenic properties of these alpha-emitting radionuclides. Such concern makes it vital that researchers use whatever means at their disposal to assess the nature and magnitude to the potential hazards and therefore provide the scientific community and the public, information about the danger of these elements in whatever form they are used. We have in mind a conference with a 5 days duration and about 50-60 participants, with the purpose of addressing the question of how current studies being conducted on thorium and radium toxicity can serve as a useful benchmark for studying and evaluating the toxicity of other alpha-emitters, namely uranium and plutonium. It will represent a truly international effort in order to bring together scientists from a number of specialities and countries, some of whom will be discussing these problems directly with each other for the first time.